Persistent phosphorescing materials, such as ZnS:Cu,Co, ZnCdS:Cu and CaSrS:Bi, have been used for many years. Recently a much brighter and longer persistent green phosphor, SrAl.sub.2 O.sub.4 :Eu.sup.2+ :Dy.sup.3+ has been produced.
Strong green luminescence from Eu.sup.2+ -doped SrAl.sub.2 O.sub.4 was reported by H. Lange in Belgian patent 1,347,45 and U.S. Pat. No. 3,294,696. Efficient luminescence in the spectral range 450-520 nm was also reported from Eu.sup.2+ -doped CaAl.sub.2 O.sub.4, MgAl.sub.2 O.sub.4, BaAl.sub.2 O.sub.4, and SrAl.sub.2 O.sub.4 and their counterparts using alkaline earth cationic combinations. (F. C. Palilla, A. K. Levine and M. R. Tomkus (1968) J. Electrochem. Soc. 115:642).
Long lasting and more efficient phosphorescence has obtained in Eu.sup.2+ doped SrAl.sub.2 O.sub.4 synthesized with excess alumina which results in formation of trapping centers associated with the Sr.sup.+2 vacancy (Abbruscato et al. (1971) J. Electrochem. Soc. 118:930).
Improved long persistence phosphors of certain alkaline earth aluminates were reported by T. Matsuzawa, Y. Aoki, N. Takeuchi and Y. Murayama (1996) J. Electrochem. Soc. 143(8):2670, and in U.S. Pat. No. 5,424,006. The brightness and persistence time of SrAl.sub.2 O.sub.4 :Eu.sup.2+ was improved by co-doping various trivalent rare earth ions to produce appropriate trapping centers. The best result was obtained by co-doping Dy.sup.+3 with Eu.sup.+2 into SrAl.sub.2 O.sub.4 and Nd.sup.3+ with Eu.sup.2+ into CaAl.sub.2 O.sub.4 to get long persistent green and purple emission, respectively. U.S. Pat. No. 5,424,006 also reports phosphors in which Mg.sup.2+ is substituted for Sr.sup.2+ in SrAl.sub.2 O.sub.4 :Eu.sup.2+, Dy.sup.3+.
EP published application 765,925 (Moriyama et al.) reports Eu.sup.2+ -activated strontium aluminate phosphors in which part of the Sr.sup.2+ of the host is replaced with Pb.sup.3+, Dy.sup.3+ or Zn.sup.2+. The zinc-doped materials are reported to display enhanced brightness and persistence compared to SrAl.sub.2 O.sub.4 :Eu.sup.2+, Dy.sup.3+.
EP published application 710,709 (Murayama et al.) reports phosphors of matrix M.sub.1 -xAl.sub.2 O.sub.4-x Where M is at least one metal selected from calcium, strontium, barium and, optionally, magnesium, and x is a number not equal to 0. The matrix comprises europium as an activator and a co-activator elected from a rare earth metal, manganese, tin or bismuth.
JP Patent 76031037 (1976, Tokyo Shibaura Electric Co.) reports blue-emitting phosphors containing barium (or calcium or strontium)-potassium (or sodium) aluminates activated with europium and manganese.
JP Patent 94029417 (1994, Matsushita Electronics) reports a strontium aluminate phosphor activated with europium modified by incorporation of yttrium oxide.
JP Patent 94029416 (1994, Matsushita Electronics) reports a europium activated barium aluminate phosphor containing yttrium oxide to enhance phosphorescence.
Zlotnikova et al. (1990) Ukr. Khim Zh. (Russ. Ed.) 56(11):1148-1151 (Chem. Abst. (1991) 115:37798k) reports composition dependence of catho-luminescent properties of a Dy-doped SrAl.sub.2 O.sub.4 --Sr Al.sub.4 O.sub.7 system.
T. R. Kutty et al. (1990) Mater. Res. Bull. 25:1355-1362 reports luminescence of Eu.sup.2+, in strontium aluminates prepared by the hydrothermal method. Blue to green luminescent phosphors of general formula Sr.sub.n Al.sub.2 O.sub.3+n where n.ltoreq.1 are reported. The reference also reports the preparation of certain aluminoborates.
B. Smets et al. (1989) J. Electrochem. Soc. 136(7):2119-2123 reports blue-emitting phosphors: 2SrO . 3Al.sub.2 O.sub.3 :Eu.sup.2+ and 1.29 (Ba, Ca)O, 6 Al.sub.2 O.sub.3 :Eu.sup.2+. In the background section of the reference the authors refer to an earlier report of blue-green emitting phosphors 4SrO . 7Al.sub.2 O.sub.3 :Eu.sup.2+ and BaO . 4Al.sub.2 O.sub.3 :Eu.sup.2+, which could be synthesized only in the presence of small amounts of B.sub.2 O.sub.3.
Chemekova et al. (1977) Terzisy Dokl. Uses. Soveshch. Rostu. Krist. 5th 2:184-185 (Chem. Abst. (1980) 93:85423h) reports synthesis of single crystals in the calcium oxide-alumina system. Addition of europium is said to produce phosphors.